Scanning receiving system



June l, 1948. l' w. H. Buss 2,442,583

SCANNING RECEIVING SYSTEM Filed oct. 1o, 1942 4 sheets-sheet 1 Tic-:I1 a.

upkyfrseoome All-0W ATTORNEY l 4 Sheets-Sheet 2 #I CoM/Meara@ June l, 1948. w. H. BLISS SGANNING RECEIVING SYSTEM Filed Oct. 10, 1942 MON/TOR June l, 1948. w. H. Buss 2,442,583

scANNING RECEIVING SYSTEM Filed Oct. l0, 1942 4 Sheets-Sheet 3 I l l l I l l l l l l l l l l .l

June 1, 1948. w. H. Buss 2,442,583

' SCANNING RECEIVING SYSTEM Filed oct. 1o, 1942 4 Sheets-Sheet 4 H Y PEC/OPO OF )VE W (SVG/V191. O/V THPE A I l I INIT/9L Our/ur oF PaLaE GENE/Paro@ #3 F/NHL OUTPUT of: Puse GEA/Een To@ #3 '1 .4. 361: 37 fl-l E I 'f F/MQL Ov7-Pur 0F #-7 COMPAQ/9702 SQURPE Pl/L SE' K Moo/F/Eo PULSE INVENTOR CnS/.9 BY /vv-w/a/ ATTORNEY Patented June 1, 1948 SCANNIN G RECEIVING SYSTEM Warren H. Bliss, Orono, Maine, assigner to Radio Corporation of America, a corporation of Dela- Ware Application October 10, 1942, Serial No. 461,526

(Cl. Z50-20) 18 Claims. l

This invention relates broadly to apparatus for scanning a given frequency band of the radio spectrum, locating any new signal which appears therein, and controlling a device for making a record of such a signal.

In monitoring the radio spectrum for enemy and illegal stations, a transmitter which suddenly goes into action, sends a short message at high speed, and then shuts down quickly is very 4hard to detect.

An object of the present invention is to provide an improved method of and apparatus Afor detecting the signals from such unknown stations, by means of which the probability that a new station will start up in such a position in a band being scanned that it will be missed by normal signal hunting is decreased to a large extent.

In brief, the present invention employs a scanning receiver -for detecting telegraph stations within the range of frequencies scanned by the receiver and recording the signals therefrom. For accomplishing this, there is provided a super heterodyne receiver which is swept or tuned rapidly over a given band of frequencies by means of a sweep device. The output of the receiver is arranged to be in the form of keyed tone whose modulations correspond to the modulations of the received signals. This keyed tone is supplied to a comparator device in which the signals of the known stations in the band of frequencies scanned by the receiver are neutralized or blanked out,-s o to speak, by locally generated pulses. To achieve this result, there are provided as many local blank-out pulse generato-rs as there are different known stations in the band to be scanned. Thus, the presence of a new signal suddenly appearing in the frequency spectrum being scanned will have no compensating or blank-out pulse and can be registered on a short loop of magnetic tape which is exactly two sweep cycles in length. This new signal from an unn known station can then be continuously recorded by means of a second receiver whose tuning is automatically adjusted by a motor control circuit to the particular position in the frequency band being scanned in which the new signal ap- In order to prevent the second receiver from responding to noise pulses which are random in nature and time ofappearance, the motor control circuit for the second receiver is controlled by a comparator device. This comparator device will Yenable the motor control circuit to function providedthe new signals are repeatedv during consecutive sweeps of the scanning superheterodyne receiver. So far, the apparatus de- 2 scribed above is in accordance with my copending application Serial No. 459,504, filed September 24, 1942, now U. S. Patent 2,409,012, granted October 8, 1946. The essential feature of the 5 present invention resides in the means which are provided for distinctly separating the signals of stations whose frequencies are close together. This is accomplished generally by modifying the usual linear rate at which the scanningL receiver is made to sweep across the band of frequencies.

An important feature of the invention lies in the system for generating a band spread pulse of proper character and combining it with the normal sawtooth sweep voltage so as to modify the normal linear sweep velocity. One result of this system is to slow up the sweep speed at one point at the sacrifice of accelerating it at another pcint. If the decelerated range occurs in a congested part of the band where there are present a plurality of signals, this will give the narrow band intermediate frequency section of the scanning receiver an opportunity to produce a much higher degree of discrimination.

A more detailed description of the invention f follows in conjunction with a drawing, wherein:

Figs. la, 1b and 1c, taken together, diagrammatically illustrate a specc embodiment of the invention;

Fig. 2 graphically illustrates the manner in which the tuning of the recording receiver is stopped at the particular portion 0f the frequency spectrum being scanned at which the newly found signals appear;

Fig. 3 graphically illustrates the operation of the 360 phase shifter of Fig. 1; and

Fig. 4 graphically illustrates the manner in which a band spread pulse of proper character is combined with the normal sawtooth sweep voltage so as to modify the normal linear sweep velocity.

In Fig. 1 of the drawing, there is shown a scanning receiver of the type generally described in my U. S. Patent 2,409,012. In fact, the circuit elements shown within boxes and labeled Superheterodyne receiver, Reactance tube, #1 comparator, #2 comparator, #1 phase shifter, #2 phase shifter, #1 pulse generator, #2 pulse generator, #3 pulse generator, Amplier, Motor control, 360 phase shifter and Monitor" are generally the 'same as those employed and described in my copending application supra. What is new, however, and forms the essence of the present invention is the addition of apparatus labeled #1 band spread generator, #4 phase shifter, #2 band spread Ipulse generator, phase shifter, and the Combining unit. These new circuit element-s function to alter the sweeprate of the scanning s-uperheterodyne receiver in the vicinity of a pulse from a known station.

Referring to Figs. la, 1b and 1c in more detail, there is shown a superheterodyne receiver I (Fig. la) indicated diagrammatically in box form. The heterodyne oscillator of the receiver (not shown) has its frequency controlled by a parallel tuned circuit 2. receive telegraph signals, and is provided with a suitable tone keying circuit for supplying keyed tone signals corresponding to the marking intervals of the received telegraph signals. These keyed tone signals appear in the output circuit 3 of the superheterodyne receiver forsubsequent use in the comparator device d (Fig. 1b) to be described in more detail later. For causing the Yreceiver to periodically scan a predetermined known type, whose anode is connected to the' tuned oscillatory circuit 2 of the heterodyne oscillator of the: receiver, 'and whose control grid is connected to the output of va sweep device E generates; sawtoothwaves; Sincel the re actance tube circuit is. well known in the frequency modulation artfor providing-a variable reactance output dependent upon the modulating voltageslapplied to the grid or thef tube, it is not believed necessary to describe this circuit further.. rhe sweep; device for generatingsaw'- tooth; oscillations comprises, inthe particular embodiment shown a gaseous tube 'I' which is arranged to rapidly discharge the condenser 8 through the space path, oi the tube after the cl'xarge. on the condenser 3: has reached a critical value.. Normally; the. condenser 8: is linearly charged from thev positive terminal. of a source of potential B+ through constant current. lirn iting pentode il.. A; negative biasl is. placed; on the grid of; trode 'l by; battery time Ehenl the voltage on @onder-ismV 8: reachesv a critical value which occurs: when the positive voltage on; con-- denser 8. overcomesthe negative bias on the grid of tubelg, a dschargepathis established through gas triode l. A variable resistor IIJI controls: the linear rate.V of chargeo-:E the condenser 8; A sixty cycle synchronizing source, labeled as such, as;-L su-res; the breakdmv-n of the eastu-he 'I` at a desired point; inV the; cycle of; operations. The;A output: of the sweep device S is connected through. lead l ii; to the grid of" the; reactance. tube 5; to( apply sixty sawtooth=` waves per second tothe; reactancev tubes.A As a result or tins... the superheterodyne receiverL willbe caused to scan. at a substantially uniform rate a. predeterniined portion of: the radio;V frequency spectrum', after whichj the re.- ceiver will return to normal and repeat the scanning operation at. the ratev or sixty complete scanning operations,1 persecond; The` rangez of the band of frequencies; tozbeswept. or scanned by the superheterodyne receiver,1 is: deterrrned to; a large.` extent bythe; magnitude,- of the. saw;

Althoughonly two generators; areV The receiver I is designed to A.

one of these, let us say the #1 pulse generator.

This blank-out pulse generator is supplied with energy from the sixty cycle synchronizing source over leads iI through phase shifter I3. The phase shifter. consists of a sixty cycle trans- Y circuiti i8.' for.' any desired portion 'of eitheror; Ity should at" this" time be'understood'that". the adjustment' ot the'Y position of the pulse, obtainablefrom the p'l'llse"V supplies negative bias for the grid of the gas' The condenser I5 across theI gas tube.

tube lli. i i is arranged to` be charged throughthe variable resistor I? and to be discharged through' the space path of the tube at a sixty cycle rate The:

voltage from condenser I 6 is suppliedto a trigger" circuit I8 composed of a pair ofrvacuumtube triodes I9 and 25'. Normally, tube 2 'isfin a state of conduction and tube i9 non-conducting.

When a pulse is impressed on' the'griid oi tub`e`|9 from condenser It, the state of conduction'of the two. tubes I9- and 26 is reversed. These two tubes, in effect, comprise a, trigger circuit which' always tends to be restored to the' condition where I-9 is non-conductive and 2B; is conductive. This restoration takes' place at a short interval after conduction starts in tube |19, and is deter mined by the values o resistor 62' and condenser" Vifhen` tube t9 becomes conducting', its plate potential immediately drops for the duration of the conduction period, as a. result of which a' pulse: of negativeypolarity is fed via l'ead 2l to*-4 the comparator 4;. At; this time, whenV tube Il? becomes conducting', tube` 2:.' will become none' conducting inf-virtue;v of. the regenerativefnature of. the feed-back' circuit'of the trigger arrange-- men-t., The valueV of; condenser S3 and the value ofV resistor t2 determines, the time.. of' conduction oi tube t9: andhence the time of duration of thel pulse impressed on: lead 2i. Cathode resistor 64 furnishes the common bias for the tubesA i9 and 2B.'

The particular time; of occurrence or positionl in the sweep cycleV of the pulseobtainabl'e from the trigger circuit I8n is controlled' by adjustment oi the phase; shifter t3 and'` more specifically by :A the adjustable resistor 21SA in` the phase shifter; Since the adjustment of the' resistor 24 of` the phaseshifter will only`- provide a variationirr po sitionA of the pulse; obtainable from the' trigger i8. overV substantially` one-habi or 180'r oi the sweep cycle, it thus becomes necessary to. provide' means for changing the position of' the pulse in the sweep cyclev overthe entire cycle and this is` accomplished by means of the reversing switchA I 2. which byf itsV operation enables theadjustable 'resist-or. 2d of; the phase shifterto control the?l positionr ot the pulsexobtainable from: the trigger the; twoV halves. of the cycle.'

grids of bothv l generator is important because this position must bemade-'to .correspond to the.A position of the signal received from a known station in order to beable to blank-out the signal from the known station-in the comparator 4'. The adjustment :of the duration of the blank-out pulse obtainable from the .pulse generator is also important because the, duration of the blank-out pulse Vmust correspond to the Yduration of the signal received from the .known station. The appearance of a blank-out ,pulse in the lead 2l will cause a relatively negative pulse to be applied to the comparator i4 vfor reasons which Aappear hereinafter.

Aspfor the comparator 4 (Figi lo) this circuit comprises .a pair lof push-pull vacuum tube yampliersli and 2G operating as class B; that nor mally biased Vto a point. of anode current cut-cti. The grids of these ampliiers are connected to opposite terminals .oi the secondary winding of `a single input transformer 21 which is designed .to pass the keyed tone appearing in the output circuit v3 or" the receiver. The combination of resistors |68 and IBI gives the proper bias for the grids of the tubes 25 and 25 by raising the cathode potential to a desired amount above ground potential. By making the cathode positive, the grid is, in effect, made to be negative since the grid is'tied down to ground potential. The anodes `of the tubes 25 and 26 of the comparator 4 are connected kin push-pull to the opposite terminals of audio output transformer 28, also designed to pass the tone. In the operation of the comparator 4, the yvacuum tubes 25` and 25 wiil normally pass the alternate half cycles of the-tone appearing inthetransformer .21` However, the application of a negative blank-out pulse frornone of the pulse generators over lead 2l will supply an ad ditional negative .bias to the grids of the tubes 25y and 2B which will prevent these tubes from passing current during the application of the blank-out pulse even in the presence of tone signals on input transformer 2l. It will thus be seen that by means of the pulse generators it is possible to prevent the comparator' from passing current at any particular time and for any desired duration in any sweep cycle.

The output of comparator 4 is passed on .to a recorder coil E5 (note Fig. 1c) which is associated with a loop of continuously moving magnetic tape 56. In this way the coil 65 registers signals on tape Eili in response to impulses which pass through comparator 4 and which are representative of a new signal.

The magnetic tape Sii is continuously driven in the direction indicated by the arrow by motor M' which is synchronously controlled by the same sixty cycle source which is associated `with the sweep device B. Loop 66 is exactly two sweep cycles in length; that is, the relative speeds of scanning of .the receiver i and rotation of loop 85 must'b'e such that the loop or tape 66 makes one complete circuit while the receiver l sweeps through accmpletc sweep cycle twice.. During each-sweep cycle of thereceiver I, a desired portion of the radio spectrum to be studied will be scanned .and the receiver' will be ready for the next sweep. A magnetic pick-up coil el is located halfway around the tape 65 from the recordin coil S5 and spaced one complete sweep cyclo apart from -coil 55 along the tape.

The pick-up coil 6l is connected to a switch 68 which in one-position (namely the left) can erase the signals recorded on the tape 5S, while in the other position (namely the right-,hand position) passes the signals picked up from loop 66 6 to the amplifier 59. The output of the amplifier B9 is in the form of amplied pulses which have been picked up by the pick-up coil El. These amplified Vpulses are passed on to a second comparator 90 where they are rectified in tube and applied to the anode of ya triode ll. The purpose .of comparator 9U is to prevent noise pulses from .being passed on to operate the motor control circuit 80. This second comparator includes va pair of rectifier tubes 'Hl and '12, and also a vacuum tube triode 1l,

Any newly found signal will appear in the output ofY transformer 28 of the comparator 4 in the form or keyed tone pulses. This newly found signal will be rectified in rectifier '2 of the second comparator and impressed upon the grid ofthe vacuum vtube trio'de 1I, at the same time that' the signal is applied to the recorder coil 65. if this newly found signal has appeared in a previous sweep cycle, it will have been registered on tape 66. When an impulse impressed on recorder coil 65 corresponds in time to an impulse registered on the tape 66 and simultaneously picked-up by coil 61, it will be seen that both rectiers 'HJ and 12 will at the same time be im` pressing positive rpotentials on both the grid and anodeelectrodes of tube 1i. This condition will cause the tube 1i to pass current, as a result 6i which a pulse of voltage will be taken cir the cathode of triode 1I and passed on to the motor control circuity 8D. The comparator S0 in this Way serves lto prevent noise from entering the motor control circuit. Since noise is fortuitous and random in character, seldom will it appear twice -in succession'at times one complete cyclev apart. Thus, if noise appears only once, there will be a pulse corresponding to this noise which will pass rectier 'l2 and be impressed upon the grid of vacuum tube 1|, but there will not be a correspondingly timed pulse picked up by coil 61, amplified by 19, rectied by tube 73 and im pressed on the anode of 1l. The application of a pulse vsolely to the grid or to the anode of tube 1I will not cause this tube to pass current since the circuit elements are so constructed and -designed that there is required the application of pulses to the anode and to the grid of rtube it simultaneously in order to cause this tube to pass current. Thus, onlywhen a newly found signal appears (which of course will be repeated during consecutive sweeps of the receiver) will the comparator pass current to the motor control circuit.

The motor control circuit 8D (Fig. lc) cornprises two gas triodes (Th'yratrons) 18 and 19, both of which are normally non-conductive. It should be noted that the path to `operate the grid of tube 'I8 comprises connection 15 and in cludes the armature and normally closed contact of relay 11. Relay T1 is in the cathode circuit of gaseous conduction tube T8 while a relay -TS is in the anode circuit of gaseous conduction tube '19. The application of a positivepulse from the comparator S0 to the lead 'I5 will cause a voltage to pass through the contacts of the relay I? and ignite tube 'i8 of the motor control circuit. rhis will cause tube 18 to pass current and operate relay 11. The operation of the relay vl1 will break the path from the lead 15 to thevgrid of .the tube 18 and will connect this path to the .grid

of tube 19. Atvthe same tlmel thev operation ofrelay 'I'IV will connect ground to lead 14 to oper ate the vspotti-ng motor. M, which, in turn, causes the conventional recording receiver. t to become operative and change its tuning .over the same being scanned will now be described. Receiver.

9| is initially tuned to one end of the Yband of frequencies being investigated. r'Ihe tripping of the motor control circuit 80 andthe consequent operation of the motor M starts the receiver 9| and causes the frequency ofthe receiver 9| to which it is tuned to change under the drive of the spotting motor. The motor M- also drives the phrase shifter 8| over shaft |82 simultaneously fwith the change in tuning of receiver 9|. Thisv phase shifter 8| is a 360 motor driven phase shifter which is synchronously controlled over leads 82 from the same 60 cycle 'synchronous source which synchronizes the sweep device E and the pulse generators, as well as synchronizing the motor M. The phase shifter 8| includes a pair of potentiometers 92 and 93 with continuous resistance elements and sliders 94 and 95 that can rotateV 360. Th'eresistance elements are tapped every 90. The two potentiometers are ganged and arranged so that, if correspond-' ing taps are in line, the sliders 94 and 95.v are mutually perpendicular. A -voltage from synchronizing leads B2 is applied through a trans# former 91 toterminals C and D on potentiometer 93, and the same magnitude of Voltage applied through 90 phaseshifter 98 and transformer 99 to terminals A and B on the other unit 92, but shifted 90 with respectv to the voltage applied to terminals C. D. The output of the 360 phase shifter 8|, taken from the'sliders 94 and 95y is fed through transformer 96 to the #3 pulse generator. Y e

The vector diagram of Fig. 3 shows the voltages at the taps of the two resistive elements. Points O and O' on both elements are all at-the same potential or reference point as shown on the vector diagram. As slider Sil-rotates, the magniev tude but not the phase of the voltage which it picks up' varies from vector' OA to vector OB and back again to OA. This'l is v'an amplitude variation with no variation in phase. Likewise the voltage fromslider 95 varies from OC to OD and back to OC. The output taken through transformer 96 is the vector sum of the slider. voltages. This output, indicated as vector OR will have a locus of Variation as indicated by the dotted line on Fig. 3.' As the common Vshaft |82 of the potentiometers 92 and 93 is rotated, the phase of'vector OR, representing the output voltage, will advance continuously in thedirection of the arrow from its initial or reference position OC and for one complete revolution of this shaft |82 the phase of the output voltage vwill change by 360.

A short pulse generator labeled -#3 pulse generator is provided which isidentical in lconstruction and operation to the'circuits of the #l 'pulse generator and #2 pulse generator. Initially, the pulse developed by this #3 pulse generator appearing in lead 83 has a phase as indicated by the pulse in line B of Fig. 2. As the phase 4shifter 8| is driven'b'y the motor M, -the position of the pulse in the output lead 83- of the #3 pulse gen-ferator advances over the sweep cycle until -such time as it coincides intime with-a pulse of the' newly found station. This is indicated in line C of Fig. 2. The pulses in lead 8 3 coming from #3 pulse generator are impressed onthe grid of tube 190i the motor control circuit 89. As long as the pulse ofth'ernewly found signal appearing'in leadi 15' does not coincide intime with the pulseap-f pvearing in lead 83, tube 19 will remain non-conductive. When, however,vv a pulse' in lead 83 coincides in time phase with a pulse appearing in lead 15 from the output of comparator 90, it" will be seenthat both pulses will be impressed on' grid ofgas triode 19 simultaneously. Since theseV pulses are positiva'the simultaneous appearance' of both pulses in leads" 15 and 83 will ignite tube 19, as a result of which relay 16'will operate. The operationof relay 16 will break the anode circuit of tube 18, thus extinguishing the are in tube 18 and causing relay 11 to release. I It is assumed, of course, that the manually operated.

switch 84 supplying positive polarizing potentials to gas tubes 18 and 19 is normally closed during all this time, it being opened only when it is 'def sired to restore the complete circuit to normal. The release of relay 11 will open its right-hand contact, thus stopping the motor M at a point which corresponds to the point at which the newly found signal appears in the frequency spectrum. The tuning of the receiver 9| will be stopped at this point in the radio frequency spectrum. The standard code recorder 85 which operates continuously from the time that receiver 9| is made to be operative by the motor M, will now record the signals of the newly found station collected by receiver 9| over antenna |92. Putting it in other words, it will be seen that since the phase of the pulse from comparator 9,9 is an indication of the positionof the new station in the spectrum,Y

and the phase of the pulse from #3 pulse generator is an indication of the frequency of the receiver9l, the matching of these pulses will match the frequency of receiver 9| to that of the de'- sired signals. -It should be noted that although receiver 9| is tuned to the frequency of the newly found signal within the spectrum being scanned,

the superheterodyne receiver I will continue scan'-` ning.

The purpose of the monitor circuit is to env able the operator to make suitable adjustments in the system. This monitor consists of a cathode ray oscilloscope 54 having horizontal deflection plates 56 and vertical deflection plates 51, tc-

gether with an associated rectifier 58 for applyeither directly to the keyed tone output circuit 3- of the superheterodyne receiver by means of lead 60, or directly to the output of the comparar tor device 4 bymeans of lead 6|. Thus; the atf tendant is able to observe at a glance by suitably operating the switch 59, the appearance' of the pulses in the output of the superheterodyne Yre-v ceiver 3 and also the appearance of the pulses in the output of the comparator 4. If the system isf properly adjusted, the key tone pulses appearing in the output of the superheterodyne receiver will not appear in the output of the comparator 4, except for the signals corresponding to thosefrom the unknown or newly found station. A

e The apparatus so far described in: detailis generally the same described in my U. S. Patent 2,409,012. The new apparatus constituting the The anode of therectifier 58 is is. to appear later. n saw-'tooth voltage developed bythe sweep de- ',vice 6,. As indicated previously, it is desirable to circuit associated with the cathode of one of the tripping tubes of |53. The tripping circuit of I t3 comprises two triode vacuum tubes I9 and 2G', the electrodes of which are respectively interconnected (as shown). The operation of this tripping circuit of IES is similar to the operation of the tripping circuit I8 of the #l pulse generator. However, the output pulse generated by the pulse generator IES is derived from the cathode of tube I9' and passed over lead IBS to the pulse reshaper consisting of condenser' IGS, inductance coil IS7,- condensers HES, it?) and resistor IIB. In order that this pulse reshaping circuit function properly, it is essential or necessary that it be driven from a low impedance source and this is accomplished by connecting the lead 65 to the cathode of tube I9. It should be noted that the tripping circuit oi H33 diers from the tripping circuit I8 of #l pulse generator in ern- 4ploying separate cathode biasing elements for vthe two tubes instead of acommon cathode biasing circuit. This is because it is desired to obtain a low impedance output from tube I9 only.

The output from the pulse reshaper is taken from resistor iii by means of a tap associated with lead iii which extends to the grid of a vacuum tube l i5 constituting the combining unit H2.

rIhe combining unit I i2 is used for the purpose or" injecting the special band spread pulses into the sawtooth wave form employed for the scanning superheterodyne receiver. The output from the combining unit I I?. is taken from the anode of the vacuum tube IIE and applied through condenser H3 to the lead I0 extending to the reactance tube circuit 5. In the operation of the system as described above, a square pulse is developed across the cathode resistor of triode I9 in a manner similar to that described for the #l pulse generator. rIhis square pulse is applied through lead m5 to the pulse reshaper where it is modified from essentially square form to a sinusoidal form, as indicated by line K of Fig. 4. This modiiied pulse is then applied oy lead HI to the grid of tube I i5 of the combining unit I I2, the output of which is further combined with saw-tooth voltage in lead I@ emanating from the sweep device The combined wave form in lead is, which is the resultant of the sawtooth wave from sweep device 6 and the modied sinusoidal or band spread pulse from the combining unit, produces a modified wave form for utilization by the reactance tube circuit.

The manner in which the band spread system of the invention functions may be understood better by referringA to the diagrams of D vto K, inclusive, of Fig. 4.

For purposes of explanation, it will be assumed that the normal output of scanning` receiver I appears as wave form D of Fig. 4, in which pulse 31 is the only signal originally pres'- ent (corresponding to the'signal from a known station) and pulse` 35 represents a signal which Wave form E is the linear -are shown well separated for purposes of explanation, they might in actual reception be combined into one irregular broad pulse because of the unavoidable poor resolving power of the narrow band intermediate frequency section of the receiver when a high sweep velocity is used.

This invention proposes to alter the sweep rate in the vicinity of pulse 37 according to the modifled saw-tooth wave G. A pulse 38 represented -in line F, having a wave form similar to one cycle of a standard sine wave, is combined with saw-tooth wave E to give the result shown in line G. In the region a-b, the normal constant slope oi the saw-tooth has been decreased to a much lower value and during this interval of time the sweep rate of the superheterodyne receiver will be less than the normal value. In the region b-d, however, the rate is much higher than normal` and in cZ--c it is below normal again.

By comparing the phase of this band spread pulse 38 with that of signal pulse 3?, it will be seen that the receiver sweep speed has been reduced on both sides of this signal pulse but increased during most of the normal pulse duration. It can be said that in the marginal areas around pulse 31 (new pulse 35 lies in one of them) velocity or sweep band spread has been accomplished while band compression has been produced during most of the original period of this pulse. As a result of this modification in sweep speed of the superheterodyne receiver, the original panoramic picture of wave form D will be reduced to that of wave form I-I. The new station pulse 33 will appear in the output of the receiver as a much larger pulse 39 because of `the better narrow intermediate frequency section resolution or build-up at slower sweep speed and pulse 3'! will be'reduced as shown to pulse because of the poorer narrow band intermediate frequency response at the high sweep rate.

As explained previously, the remaining pulse 'ii may be balanced out of the #l comparator 5 output by means of a locally supplied counter or blank out pulse from #l pulse generator.

`When a new signal 36 appears on a frequency very close to an original signal 3'?, it will be well separated therefrom (as shown by pulse I of the wave form I) and can be detected as describedA in the disclosure referred to.

In Fig. 1 a multiplicity of band spread pulse generators #l and #2 are shown, so that this feature may be had for each of the originalsignals found in the band. It is also quite possible to vhave band spread pulses different in wave form from that shown in wave form F, Fig. 4, which was given only by way of example.

Lines J and K of Fig. 4 are given as a suggestion of a circuit operation for developing a wave form similar to that of pulse 38, line F. If a rectangular or square pulse such as that of wave form J is applied to the input of the wave reshaper circuit of Fig. 1, a single cycle damped sinusoid like that of wave form BI will result if elements IUS, |61, IS, it* and liti of Fig. 1 are of the proper nature relative to each other.

The reason why the superheterodyne receiver can give a much higher degree of discrimination between closely spaced signals if the usual linear scanning rate of the receiver is modified, may be better understood by an appreciation of the principle Athat the ability of the narrow band pass intermediate frequency selective circuit in the receiver to build up a signal is generally inversely proportional to the sweep rate; il e., the slope of the saw-tooth Wave. The rate of sweep which apparatus-for periodically and continually tuning said' receiver over a desired g radio lfrequency band, said circuit apparatus including means for tuning the receiver at a substantially linearrate over one part of said band and electronic means for modifying the substantially linear rate of tuning over another part of said band as the receiver tunes from one end of the band to the other end, to thereby produce a receiver response characteristic over said one part of the band whichis different from the receiver response characteristic over said other part of the band.

3, In a receiving system, a receiver, means including an oscillator of saw-tooth waves for repeatedly tuning the receiver at a substantially linear rate over a selected band of frequencies, circuit apparatus for producing substantially sinusoidal waves of shorter duration than said saw-tooth waves but of the same frequency, and means for combining said sinusoidal -waves with said saw-tooth waves to modify the normally substantially linear rate of tuning the Yreceiver over said band, to thereby change the response characterof the receiver over diiferent portions of said band of frequencies. 4. In a receiving system, a receiver, electron discharge device apparatus for repeatedly tuning the receiver at a substantially linear rate over a selected band of frequencies, a wave form oscillator synchronizedin frequency with the tuning rate of said receiver, and means for combining the output of said oscillator with the output of said electron discharge device apparatus for modifying lthenorrnally substantially linear rate of tuning said receiver at any desired point over the selected band of frequencies being scanned, to thereby change vthe response character of the receiver at that point compared to other points in said selected band of frequencies. l

5. In a receiving system, a receiver, an electron Ydischarge device coupled to the output of said receiver, means for periodically tuning the receiver over a selected band of frequencies, a .local pulse generator producing pulses of predetermined polarity, means for combining the pulses from said generator with the output of said receiver, whereby a signal from a known station appearing in the output of said receiver may be prevented from passing through said discharge device, and means for modifying the rate of tuning' oi' the receiver at that portion of the band at which the signal from the known station appears.

6. In a receiving system, a receiver, an output circuit for said receiver, means for periodically tuning the receiver over a selected band of Af-requencies', a local pulse generator, a circuit for adjusting the positionv of the pulse produced by said generator during the cycle of operation, means for Ycombining the pulses from said generator with the output of said receiver, whereby a signal from a known station appearing in the output of said receiver may be prevented from passing through said' output circuit, and means for modifying the rate of tuning of the receiver at and adjacent to that portion of the band at which the signal from the known station appears.

'7. In a receiving system, a receiver, an output circuit for said receiver, means for periodically tuning the receiver over a selected band of frequencies, a local pulse generator producing unidirectional pulses, a circuit for adjusting the position of the pulse produced by said generator during a cycle of operation, means for combining the pulses from said generator with the output of said receiver, whereby asignal from va known station appearing inthe output of said receiver may be prevented from passing through said output circuit, and means for modifying the rate of tuning of the receiver at those points in the band corresponding to the marginal edges of the signal of the known station.

8. In a receiving system, a receiver, a utilization circuitrfor,said-receiver, means for periodically tuning the, receiver over a selected band of frequencies, a localpulse generator, means coupled to said pulse generator and responsive to the pulses produced thereby for preventing a signal from a known station appearing in said receiver from being passed on to said utilization circuit, and means for decreasing the normal rate of tuning of the receiver at those points in the band corresponding to the marginal edges ofthe signal from the knownstation.

9. In a receiving system, a receiver, a utilization circuit for said receiver, means for periodically tuning the receiver over a selected band of frequencies, a ylocal pulse generator, means coupled to said pulse generator and responsive to the pulses produced thereby for preventing a signal from a known station appearing in said receiver from being passed on to said utilization circuit, and means for decreasing the normal rate of tuning of the receiver at those points in the band corresponding to the marginaly edges of the signal from the known station and for increas ing the normal rate of tuning of the receiver at that point in the band corresponding to the center of the signal from the known station.

10. In a receiving system, a receiver, means including a saw-tooth oscillator for repeatedly tuning the receiver in substantially linear manner over a selected band of frequencies, a local pulse generator producing unidirectional pulses of predetermined polarity, said pulses having a requency the same as said saw-tooth oscillator, a circuit for controlling the time of occurrence of the pulses produced by said local pulse generator during the cycle of operations, and a circuit connecting the outputof said generator with the output of said saw-tooth oscillator, whereby the normally substantially linear rate of tuning of said receiver can be modified at any desired portion of said band with a'consequent modification of the character of the receiver response at that point.

1l. In a receiving system, a receiver, means including a saw-tooth oscillator coupled to an electrode of a reactance tube for tuningv the receiverinsubstantially linear manner over a selected band of frequencies, a local pulse generator producing :pulses having a frequency the same as said saw-toothoscillator, a circuit for controlling the time of occurrence of thepulses produced 4by said local -pulse generator, and an electron discharge device combining unit coupling the output of said generator to the same electrode of the reactance tube to which said saw-tooth ,oscillator -is connected; vwhereby the normally substantially linear rate of tuning of said receiver can be modified at any desired portion of said band with a consequent modification of the character of the receiver response at that point.

12. In a receiving system, a receiver, a utilization circuit for said receiver means including a saw-tooth oscillator for periodically tuning the receiver over a selected band of frequencies, a local pulse generator, means coupled to said pulse generator and responsive to the pulses prois ducedtherebr 'forA prever'tin 'al-sfl'a fregi; al` kriewlr st'ciefappeerirfg inf sal-c1 receiver from beingpassed olil @o said? utiliioxr eiru'g another local puise- 'gericaccn anetwck ne ucffet of said last puIse generator f' reshaping' the Wave fom of the pulse generated 5y said Vlas?, pulse generator, means forlr i'lchoiing the pulse rate of both said puise' generators W'h'the frequency of said seu/v'-foothA osc'ili'aofrand a 'circuitV for o'cliiilcinrfrg.` the output of said 'iretvrork with the outputA of said 'savv=t0bth= oscillator for modifying th rat' of 'filming the bvf at a desired point in the bad of frequencies being scanned;

13.- I cornicination,` a recever, a caliode ray device' coupled to tzi'ievc'lutlutl of said receiver, a surce of waves coupled to Said device' fdr prot duc'ing alinear sv/ep o'f tie ray of 'said device and also coufedtaid fivf io pel'odally time said receiver over a selected band of fieq'ueri'cies,v a wave form! os'ill'aeor iiav'iiig aA fre-f qu'n'cy the Sme' als Said SQUEQ; 'nd' rfli's fi combining the output;- f said oscilatol vvth' said source for changing the I'I'Oif a 'tullg of saidrece'ver and for modifying the lineari rat of sweepv of said ray ray vfrlcui's iii ie dr'ectiin fn' One e' E6 the ohel" nc f its path of travel.

15; Iri af receiving systelii,Y a receiver; n'iaiisfof peridica-HY tuning tli' rie offer IeCd band of frequencies; tube cupiedto the output of said receiver, ael'erar' rodc;

ing unidirectional nurses of 'relatively' negative polarity; a cdnnecto'n fror the utput of said generator to` said vacuufl'l" tlub'e, whereby the pulses supplied by said 'generator bias said vacuum tube to prevent the passage of signals therethrough for durst70ri of the pill-ses,- a circuit for adjusting the position of said pulses during the cycle of operatioi, arid means' including another uis generator' for' irrodfyingthe rate of tuning of the" receiver' at a desrd'point in the se'icted band of freduenees being scanned.y

16. In al receiving systei; a receiver, a utilizati'en circuit forsaid receiver, nlans fo periode icalIy tuning tlfie receiverveil a sgeetec band of frequencies', a Ideal' pulse @euere-tor operating? ln" sychonisr'n Wih said means,` a eiiluit 'for adjusting the os'itonof the pulse lrddud 'Ey eli-misceing the receiver ver'said Toa-nd,- fo thereby Change ie cfl this patent:

UNrTED STATES PATENTS Y Niimpeif l Name Dare' 1,774,146 Let' Alle: 26,` 1930 131233? Walac'e Sept, 20;, 1932 1,883,912? Y Halrhawy 406225,11932 1,917,268 Mirick July 11,l '1933` 1,932,925 Chauveau Y O ct. 31,19133 M9423? Schlick Mar. 12, 1935 (137,571- Harr'ies VApr. 14, 1936 2,056,220 Lowell -10ct. 6,. 193B 2,053,534 Wallace 'Dfec 8, 1926 2,279,151 Wallace Apr. '1, 1942' 2,237,925 'White June 30', 1942' 23011189 Wolff c't. 27,l 1942 2,312,203 Wallace 1 Feb. 23, 19451 2,409,012 Bliss Oct.; 8, 1246 2,411,494 i Bliss 1 Nov. 19, 1946 

